Lithography (also known as photolithography) is a commonly used technique in the integrated circuit manufacturing process. In a lithography process, a material layer that is to be patterned is formed first. A photo resist is applied on the material layer. The photo resist is exposed to light through a lithography mask, which includes opaque patterns that allow the light to pass through, and transparent patterns that block the light. The exposed photo resist is developed to form a patterned photo resist. In the patterned photo resist, depending on whether the photo resist is negative or positive, either the portions that were exposed to the light are removed, or the portions that were not exposed to the light are removed. The material layer is then etched, wherein the portions of the material layer protected by remaining portions of the photo resist remain, and the un-protected portions of the material layer are removed.
In order to performing the lithography processes, the lithography mask needs to be made first. The formation of a lithography mask includes forming an opaque layer over a transparent substrate, and then patterning the opaque layer. The regions where the opaque layer has portions left are the opaque portions of the resulting lithography mask, and the regions where the opaque layer is removed are the transparent portions of the resulting lithography mask. In the patterning of the lithography mask, however, defects may occur. For example, some portions of the opaque layer intended to be removed may actually be left un-removed. These portions need to be removed in a repair process.
In the repair process, an electron beam or an ion beam is used to scan and remove the defective portions. The stop point of the repair process, which should be the time point that the undesirable portions of the opaque layer are removed and the underlying transparent substrate is removed, needs to be determined. In conventional stop point detection methods, the end point detection was made by detecting the secondary-electrons, and using secondary-electron signals to visually determine whether the end point is reached or not. The determination of the stop point may also include finding a sharp drop of the secondary-electron signals, finding a low secondary-electron signal that occurs after the sharp drop, and then calculating the stop point by multiplying the low point with a constant. Since the low point is rather ambiguous and suffers from human errors and the fluctuation in the secondary-electron signals, the determination of the stop point is not accurate.